Analyzers such as mass spectrometers or optical spectrometers are commonly used to perform chemical analyses. The most common form of sample used in such analyses is a liquid, often formed by dissolving the sample in dilute acid. The liquid sample is then sprayed into a hot plasma which largely vaporizes, atomizes and ionizes the elements in the sample. These elements then can be detected either by optical emission spectroscopy or by sampling the ions into a vacuum chamber for mass spectroscopy.
Commonly, pneumatic or sometimes ultrasonic nebulizers are used to introduce samples into plasmas. The nebulizers produce a fine spray having a distribution of drop sizes. A small portion of this distribution, consisting mostly of the smallest drops, is selected by a settling chamber and carried into the plasma by a sample gas stream (commonly argon), where it is vaporized by the plasma. This method, although commonly used, has many disadvantages. One disadvantage is that it produces a noisy signal, due to the statistics of both drop size distribution and drop arrival time and position in the plasma. Another disadvantage is that oxide interferences (unwanted oxide compounds which complicate spectral interpretation) result from the solvent (commonly water) which evaporates and provides oxygen in the plasma. This can be mitigated by interposing heating and drying but conventional methods of implementing these steps add complication and deleterious effects (e.g. memory effects and sample loss).
A further disadvantage of the current methods is that nebulizers waste most of the sample, typically 95%. This is undesirable where only a limited quantity of sample is available, and it creates an increasingly important problem in sample handling and safe disposal of the acid waste when large amounts of acids and samples are involved. Finally, nebulizers, spray settling chambers and dryers all contribute to increased washout time, the required time between different samples to avoid cross-contamination (the so-called memory effect). This washout time reduces the productivity of the instrument.
Other approaches have been described to deal with some of the above problems. Trains of uniform sized (i.e. mono-dispersed) liquid droplets have been injected into the plasma to facilitate the study of the underlying processes of evaporation and vaporization. The droplet sizes used (60 to 80 microns in diameter) were considered to be too large for analytical usage, so a variant in this theme added a strong shear flow to shatter these droplets into smaller droplets.
Another approach which has been described utilizes a micro nebulizer which disperses the totality of a much smaller liquid sample in nebulized form directly into the plasma. This eliminates sample wastage and reduces memory effects and so is more suitable for coupling to automated sample injectors or to liquid chromatograph columns. However unlike the present invention (as will be described), the sample is injected into the plasma while still in liquid form, and poly-dispersed, with attendant disadvantages as will be clear from the following description.